No abstract

T wo germinative zones of the telencephalon, the dorsal (pallial) and the ventral (subpallial) ventricular zones (VZ), provide new neurons to the cerebral cortex. Postmitotic cells from the dorsal VZ differentiate into excitatory projection neurons after a radial migration whereas the ventral part of the VZ, including the lateral (LGE), medial (MGE), and caudal ganglionic eminences, generates neurons of the basal ganglia and cortical interneurons (1-7). The cortical neurons, born early to form the preplate, are thought to function as a scaffold for the assembly of the cortical architecture (8-12). Other cortical neurons, born later, migrate radially and insert themselves into the preplate to generate the cortical plate (CP), which splits the preplate into the marginal zone (MZ) and the subplate (SP) (8, 11).Cajal-Retzius cells, which reside in the preplate and thereafter in the MZ, are considered to be the earliest neurons to differentiate in the developing neocortex (8). These cells secrete reelin (13-19), an extracellular matrix glycoprotein that is crucial for cortical lamination (20,21). Pioneer neurons are also preplatederived; they guide thalamocortical afferent axons into the cortex and cortical efferent axons to their subcortical targets (10, 11). Recently, we described a population of pioneer neurons in the MZ (17,18,22,23), but their origins, their molecular properties, and their functions remained unexplored (12,24). We here characterize the MZ pioneer neurons by combining cell tracers and cell-specific markers with patch-clamp recordings. We found that the adhesion molecules TAG-1 and L1 are reliable markers of such neurons. We show that MZ pioneer neurons migrate tangentially to the MZ from subpallial sources. We demonstrate that such a migration is reelin-dependent and, in addition, that reelin controls the arrival of MGE-derived interneurons to the CP during early corticogenesis. Materials and MethodsOrganotypic Slices. Brains were obtained from embryos of pregnant OF1 mice at embryonic day 14 (E14) (Iffa Credo; n ϭ 64; date of plug ϭ E0). All animals used in this study were maintained and treated according to protocols approved by the authors' institutions. Dams were killed by cervical dislocation under Isof lurane anesthesia (Belamont, Neuilly-sur-Seine, France). The uterus and embryos were removed and rapidly isolated in cold oxygenated (5% CO 2 and 95% O 2 ) artificial corticospinal fluid (in mM): 124 NaCl, 3 KCl, 1.3 MgSO 4 , 26 NaHCO 3 , 1.25 NaH 2 PO 4 , 10 glucose, and 2 CaCl 2 . Brains were embedded in 4% agar and coronally sliced (300 m) with a vibrating microtome (VT-1000, Leica, Argenteuil, France). Slices were placed onto Millicell-CM membranes (Millipore, Bedford, MA) in 35-mm Petri dishes containing 1 ml of the following medium (Life Technologies, Cergy Pontoise, France): 50% basal medium with Eagle's salts, 25% horse normal serum, 25% Hanks' balanced salt solution, 4.5 mg͞ml D-glucose, and 0.1 mM L-glutamine (25). The medium had been heated previously at 56°C for 30 min to inactiva...

The calcium binding protein parvalbumin (PV) is widely distributed in the mammalian nervous system and its relationship with GABAergic neurons differs within thalamic nuclei and animal species. In the rat somatosensory thalamus PV immunoreactive (ir) neurons were found only in the GABAergic reticular thalamic nucleus (RT), while a dense PVir neuropil is present in the ventrobasal complex (VB). In this study the distribution and relationship of PV and GABA were investigated in RT and VB during postnatal development at electron microscopic level. The pre-embedding immunoperoxidase detection of PV was combined with the post-embedding immunogold localization of GABA. In RT, at all developmental ages, neuronal cell bodies, dendrites and rare axonal terminals were both PVir and GABAir. In VB during the first postnatal week several small vesicle-containing profiles were double-labelled and some of them were identifiable as synaptic terminals. From postnatal day 7 (P7) to P9 the medial part of VB was more intensely PVir than the lateral one and some differences in the sequence of maturation of PVir terminals were noted between these two VB subdivisions. Single-labelled PVir profiles were first observed at P8, whereas single-labelled PVir terminals appeared at P12 and at P15 they became more frequent and larger, showing the typical morphology of ascending afferents described in adult VB. These results demonstrate the late expression of PV and acquisition of adult morphology in ascending terminals of rat VB during postnatal development in comparison with the innervation arising from the GABAergic RT.